1. Field of the Invention
The present invention relates to a clamping mechanism that secures a workpiece to a mechanical arm. More particularly, the present invention relates to a clamp that gently secures a semiconductor wafer to a robot blade by biasing the wafer against a retaining member at the forward edge of the blade when the robot blade is at least partially retracted for rotation.
2. Background of the Related Art
Modern semiconductor processing systems include cluster tools which integrate a number of process chambers together in order to perform several sequential processing steps without removing the substrate from a highly controlled processing environment. These chambers may include, for example, degas chambers, substrate preconditioning chambers, cooldown chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, etch chambers, and the like. The combination of chambers in a cluster tool, as well as the operating conditions and parameters under which these chambers are run, are selected to fabricate specific structures using a specific process recipe and process flow.
Once the cluster tool has been set up with a desired set of chambers and auxiliary equipment for performing certain process steps, the cluster tool will typically process a large number of substrates by continuously passing substrates through a series of chambers and process steps. The process recipes and sequences will typically be programmed into a microprocessor controller that will direct, control, and monitor the processing of each substrate through the cluster tool. Once an entire cassette of wafers has been successfully processed through the cluster tool, the cassette may be passed to yet another cluster tool or stand alone tool, such as a chemical mechanical polisher, for further processing.
Typical cluster tools process substrates by passing the substrates through a series of process chambers. In these systems, a robot is used to pass the wafers through a series of processing chambers. Each of the processing chambers is constructed to accommodate and process two wafers at a time. In this way, throughput of substrates in the cluster tool is effectively doubled. The amount of time required by each process and handling step has a direct impact on the throughput of substrates per unit of time. While the exact design of an integrated circuit fabrication system may be complex, it is almost always beneficial to perform each step as quickly as possible to maximize overall throughput without detrimentally affecting product quality, operating costs, or the life of the equipment.
Substrate throughput in a cluster tool can be improved by increasing the speed of the wafer handling robot positioned in the transfer chamber. As shown in FIG. 1, the magnetically coupled robot comprises a frog-leg type connection or arms between the magnetic clamps and the wafer blades to provide both radial and rotational movement of the robot blades in a fixed plane. Radial and rotational movements can be coordinated or combined in order to pick up, transfer, and deliver substrates from one location within the cluster tool to another, such as from one chamber to an adjacent chamber.
Another exemplary robot is shown in FIG. 2. FIG. 2 shows a conventional polar robot with an embodiment of the substrate clamping apparatus of the present invention. As shown in FIG. 2, like the "frog-leg" type robot of FIG. 1, radial and rotational movements may be coordinated or combined in order to pick up, transfer, and deliver substrates from one location within a cluster tool to another, such as from one chamber to an adjacent chamber. However, unlike the robot in FIG. 1, the robot shown in FIG. 2 may also provide translational movement of wafer 302.
As the robot speed and acceleration increase, the amount of time spent handling each substrate and delivering each substrate to its next destination is decreased. However, the desire for speed must be balanced against the possibility of damaging the substrate or the films formed thereon. If a robot moves a substrate too abruptly, or rotates the wafer blade too fast, then the wafer may slide off the blade, potentially damaging both the wafer and the chamber or robot. Further, sliding movements of the substrate on the wafer blade may create particle contaminants which, if received on a substrate, can contaminate one or more die and, thereby, reduce the die yield from a substrate. In addition, movement of the substrate on the wafer blade may cause substantial misalignment of the substrate that may result in inaccurate processing or even additional particle generation when the substrate is later aligned on the support member in the chamber.
The robot blade is typically made with a wafer bridge on the distal end of the wafer blade that extends upwardly to restrain the wafer from slipping over the end. However, the wafer bridge does not extend around the sides of the blade and does very little to prevent the wafer from slipping laterally on the blade. Furthermore, the wafers are not always perfectly positioned against the bridge. Sudden movement or high rotational speeds may throw the wafer against the bridge and cause damage to the wafer or cause the wafer to slip over the bridge and/or off the blade.
There is a certain amount of friction that exists between the bottom surface of a wafer and the top surface of the wafer blade that resists slippage of the wafer. However, the bottom surface of a silicon wafer is very smooth and has a low coefficient of friction with the wafer blade, which is typically made of nickel plated aluminum, stainless steel or ceramic. Furthermore, a typical wafer is so lightweight that the total resistance due to friction is easily exceeded by the centrifugal forces applied during rapid rotation of the robot, even when the blade is in the fully retracted position. However, this low coefficient of friction is typically relied upon when determining the speed at which a robot rotates.
Patent application Ser. No. 08/935,293, entitled "Substrate Clamping Apparatus," filed on Sep. 22, 1997, which is hereby incorporated by reference discusses the problem of wafer slippage on a robot blade and the need to increase wafer transfer speeds. This application describes a clamping mechanism that holds the substrate on the blade during transfer. However, that invention is directed to a complex lever/flexure system to engage and disengage the clamp fingers.
There is a need for a robot that can transfer wafers at increased speeds and acceleration/decelerations, particularly in a multiple or single substrate processing system. More specifically, there is a need for a wafer clamping mechanism on a robot that can secure a wafer or a pair of wafers on a wafer blade or a pair of wafer blades with sufficient force to prevent wafer slippage and wafer damage during rapid rotation and radial movement.